Synthesis of Carbene-Stabilized PNPN Fragments and Their Carbene-Dependent Redox Properties.

Herein, we report the synthesis of carbene-stabilized 1,3-diaza-2,4-diphosphabutenes CAAC Me PNPNCAAC Me 4 CAAC (CAAC Me = 1-[2,6-bis(isopropyl)phenyl]-3,3,5,5-tetramethyl-2-pyrrolidinylidene) and IPrPNPNIPr 4 NHC (IPr = 1,3-Bis(2,6-diisopropylphenyl)-imidazol-2-ylidene). The bonding in both systems is defined by a delocalized polar covalent π-system, with 4 NHC exhibiting increased conjugation relative to 4 CAAC . The nature of the stabilizing carbene also influences the redox properties of the compound, with 4 CAAC undergoing potassium-mediated reduction to the closed-shell P-P bonded dimer K 2 5 2 , which upon treatment with Kryptofix-2,2,2 converts to the transient radical anion [K crypt ][ 5 ], the formal one-electron reduction product of 4 CAAC . In contrast, 4 NHC undergoes reversible one-electron oxidation to the stable radical cation [ 6 NHC ][SbF 6 ]. Computational and spectroscopic analyses of both radical species are suggestive of unevenly delocalized spin, with the bulk of the spin density residing on phosphorus in both cases.